README: point package upgrade link to new location

The package upgrade link points to a file that states the
information has moved to a new location, and that the file will
be removed at the end of 2024 Q1. Update the link in the README
to point directly to the new location.

BUG=b:305810910
TEST=None

Change-Id: I81441d33bbb805098f5444c665d5833e1c285b90
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/5366140
Reviewed-by: Anuj Jamwal <anujjamwal@google.com>
Commit-Queue: Bob Haarman <inglorion@chromium.org>
Tested-by: Bob Haarman <inglorion@chromium.org>
1 file changed